Lecture 8: Precipitation and Evaporation Reading: Applied Hydrology Sec on evaporation and evapotranspiration

NARR Energy Budget Revisited Net radiation, Rn, at the land surface is the energy provided by the difference between incoming and outgoing radiation This energy is consumed by snowmelt, and ground, sensible and latent heat fluxes UpSW UpLW DnSW DnLW Rn = DnSW + DnLW – UpSW - UpLw SH LE SPH Melting of snow cover G Rn + LE + SH + G + SPH = 0

Precipitation Precipitation requires air mass lifting. This can occur in 3 ways: –Convective cells –Fronts –Orographic (mountains) –Or by a combination of the above

Rising Air Cools As air rises, it cools “adiabatically” that is it does not exchange heat with its surroundings (they are cooling similarly) Lapse rate ranges: –~6°C/km Wet –~10°C/km Dry

Water droplets form in clouds

Water drops fall with a terminal velocity Generally D < 3mm F g = weight of drop F b = bouyancy force on drop F d = drag force of air on drop At equilibrium F g – F b – F d = 0

Terminal Velocity

Thunderstorms can be analyzed using a control volume model

Spatial Variability of Precipitation 3 hour forecast precipitation, July 2003 This is a very complex subject that the NARR does a better job of depicting than books and maps.

Forms of Precipitation Precipitation Liquid (Rainfall) Solid (Ice) Snow (vapor condensed to ice) Hail (water condensed to ice) We are going to concentrate on rainfall and not worry about snowfall, snowmelt, etc (which are very important subjects in the North and West of the US)

Rainfall Hyetograph The graph of rainfall vs time at a point is called the rainfall hyetograph

Extreme Rainfall

Evaporation Evaporation happens in several ways –Open water evaporation –Transpiration from leaves –Evaporation from soil and land surface Evapotranspiration

Factors affecting evaporation Heat energy to supply latent heat of vaporization (Net radiation, air temperature) Capacity to transport vapor away from evaporative surface (wind, humidity) Water available to supply evaporative moisture (soil water content) Potential evaporation is evaporation when water supply is not limiting

Evaporation Pan

Effect of Vapor Transport